The present invention relates to the control of electric arc furnaces, such as melting furnaces, and more particularly to a control system for automatically positioning the electrodes to maintain the desired current and arc conditions.
Arc furnaces have been automatically controlled heretofore by controlling the electrode position in accordance with the arc circuit impedance. This may be done by sensing the arc current and the arc voltage and moving each electrode as required to maintain a fixed ratio of voltage-to-current corresponding to the desired impedance. Thus, in Roberts Pat. No. 3,597,518, a system is shown in which a signal representing the arc current in an electrode is balanced against a signal representing the arc voltage to provide an error signal when the current and voltage signals become unequal, indicating a departure from the set value of arc circuit impedance. The error signal is then applied to an integrating type of regulating system to derive an analog command signal for the motor controller to raise or lower the electrode as required to reduce the error signal.
The system of the Roberts patent is in successful use but it has the disadvantage that the speed of response is influenced by the resistance of the arc circuit, and is relatively slow at low resistance values. This is for the reason that the arc circuit contains an appreciable amount of reactance, which is relatively constant as it is determined primarily by the cable runs and the mechanical structure and arrangement of the electrode. The impedance of the circuit is, of course, a function of both resistance and reactance and if the resistance is relatively high, that is, if the resistance is greater than the reactance, variation in resistance substantially changes the impedance of the circuit and arc regulation based on impedance operates satisfactorily. If the resistance is less than the reactance, however, a change in resistance has less effect on the impedance and regulation based on impedance sensing becomes increasingly insensitive as the arc resistance decreases. This results in relatively low error signals at low arc resistance and correspondingly slow response of the regulator system to position the electrodes in the desired manner. Control of the electrode position in response to the arc circuit resistance rather than the impedance would, therefore, result in faster and more accurate response of the regulating system.